1. Field of Invention
This invention relates to systems and methods used for providing millimeter wave and sub-millimeter wave illumination of a scene. In particular, it relates to illuminators that are used primarily indoors, but also have utility in outdoor environments.
2. Discussion of Prior Art
Systems capable of producing images of a scene based on millimeter wave (or thereabouts) radiation emitted from the scene are becoming more commonplace as the technology used in such systems matures. These imaging systems are able to detect the energy levels at these wavelengths within a scene at a resolution fine enough to produce images of a quality suitable for many tasks. The images produced from such imagers effectively show the energy differences coming from different parts of the scene. To satisfactorily reproduce various elements of the scene there needs to be a sufficient illumination energy difference between these elements. If the system is being used outdoors then the energy difference between the sky, which appears cold and hence of low emissivity, and the scene, which will be typically at 270-300 Kelvin and hence of comparatively high emissivity, can be used to generate images having a good contrast. For indoor use, this is more of a problem, as typically all elements within the scene will be at a similar temperature.
In these circumstances it is normal to direct a source or sources of energy at the wavelength(s) of interest at the scene to be imaged, such that the scene may reflect some of this energy to the imager, and so distinguish the scene from its background. An illuminator designed for this purpose is disclosed in published PCT application, No. GB2003/001886. The illuminator disclosed therein provides a diffuse source of energy from a relatively large area, which results in a more even illumination of the scene being imaged, and helps to reduce problems such as glint and specular reflection, all of which are detrimental to the final image.
However, if the bandwidth of either the illuminator or the imager is small, then a speckle effect can sometimes be seen on the image. This can be detrimental to the overall image.
The object of the present invention is to at least alleviate the problems identified with the prior art.